The present invention relates generally to controls that monitor the feeding of coil stock between reciprocating dies of a mechanical power press. More particularly, the invention relates to an adjustable coil stock feed check point which may be selectively used to interrupt coil stock feed and the press cycle prior to the dies contacting the coil stock.
Mechanical power presses include hydraulic presses, crank driven power presses, press brakes, shears and other machinery for working coil stock by continuously executing and repeating a coil stock feed cycle. Referring to a crank driven power press, for example, has a rotating crankshaft with a horizontal axis of rotation in which the eccentric of the crankshaft is connected by a connecting rod to a die. Typically, an electric motor is connected to a fly wheel through a clutch and brake which is selectively engaged and disengaged to rotate the crankshaft through a full 360.degree. revolution. With each revolution of the crankshaft, the moveable die is reciprocated through one complete cycle moving toward and away from a stationary die. A coil stock feed device is coupled to the rotating crankshaft of the press or, is otherwise independently powered, to incrementally translate coil stock material between the moveable and stationary dies. The translation of the coil stock is synchronized with the rotation of the crankshaft.
A point on the eccentric of the crankshaft moves through a circular path in a vertical plane which has four reference points of interest. A top, or 12 o'clock, reference point, also known as the top dead center point, is the point at which the eccentric on the rotating crank and the reciprocating die attached thereto are at their highest position with respect to the frame. A bottom, or 6 o'clock, reference point, that is, the bottom dead center point, is the point at which the eccentric and the reciprocating die rotatably attached thereto are at their lowest position with respect to the frame. At the bottom reference point, the reciprocating die is closest to and may be in contact with the stationary die and is effective to perform work on the coil stock located between the dies. Typically, the top and bottom reference points are also defined by the intersection points of a vertical diameter across the circular path of the eccentric of the crankshaft.
In a typical production cycle of operation of the press, an upstroke reference point is selected as a material feed start point, that is, the point at which the coil stock feed device initiates an incremental translation of the coil stock between the moveable and stationary dies. The eccentric on the crankshaft passes the top reference point and moves downward along the circular path to a downstroke reference point which is selected as a material feed stop point, that is, the point at which the coil stock feed device terminates the incremental coil stock feed through the dies. The exact locations of the material feed start and stop points relative to the circular path of the eccentric is a function of the desired length of material feed, the type of press feed being used, the application and to some extent the discretion of the operator setting up the press cycle. The coil stock may start feeding at any point after the dies and pilots clear the coil stock on the upstroke. The feed is typically completed at the 3 o'clock position, but it may be completed at any point prior to the coil stock being contacted by the dies. After the coil stock feed is stopped, as the eccentric of the crankshaft continues its rotation toward the bottom reference point, the dies come closer together; and pilot pins or other devices on the dies precisely locate the coil stock within the die prior to the dies reaching a working position, that is, any position of the dies relative to the coil stock at which the dies are in contact with the coil stock and performing work on the coil stock. Most typically the working positions of the dies occur when the eccentric on the crank is close to the bottom reference point.
The above described production cycle is operated at a high repetition rate, and a misfeed of the coil stock can cause substantial damage and economic loss. Therefore, there are devices which measure a feed increment, detect a misfeed of the coil stock and stop the production cycle of operation in response to detecting a misfeed. For example, in U.S. Pat. No. 3,393,589, a measuring device is disclosed which measures the length of feed of the coil stock during a cycle of operation of the press. If the feed is inaccurate, a signal is produced which is effective to stop the feed cycle. With the '589 patent and other coil stock feed measuring devices, a signal to stop a cycle of operation is not generated until the end of the coil stock feed which corresponds to the point when the eccentric of the rotating crank is at the downstroke reference point. Typically, the press is designed to stop the crank when the eccentric is at the 12 o'clock, top reference point so that gravity can be used as a decelerating force when the brake is engaged. Therefore, after a coil stock misfeed is detected, the dies in the press are brought together and pass the 6 o'clock, down stroke reference point at least one time prior to the press stopping at the at the next 12 o'clock, top reference point.
The above cycle of operation has the disadvantage of bringing the dies together and executing at least one working stroke after a misfeed has been detected. The extent of damage from a misfeed depends on the extent to which the coil stock is misfed, die design, and how quickly the problem is detected. In some cases, the press can continue to operate without damage to the dies, but it produces parts which are out of tolerance and scrap. In other cases, with slight misfeeds, the pilots and die punches, which are frequently made of a brittle carbide material, may be broken. More severe damage to the dies may also occur; but in either case, any damage requires that the press be taken out of production, the dies removed therefrom, taken to a tool room for reworking and repair, and then returned and reinstalled on the press. The press may or may not be idle during the time of die repair, however, the above represents a significant economic loss to repair damage that might otherwise be preventable.
In view of the above, it would be preferable to stop the feed cycle of the press prior to the dies or the pilots on the dies contacting the coil stock. Production rates of presses range from approximately 10 to 600 strokes of the reciprocating die per minute. Assume for purposes of illustration that the coil stock stops feeding at the 3 o'clock position and the dies contact the coil stock at the 6 o'clock position. At a typical press production rate of 120 strokes per minute, the period of time between the eccentric and the crankshaft passing the 3 o'clock, downstroke reference point and reaching the 6 o'clock, bottom reference point is approximately 125 milliseconds. However, after detecting a coil stock misfeed, when the eccentric of the crankshaft passes the 3 o'clock, downstroke reference point, a typical electronic control requires from 10 milliseconds to 50 milliseconds to detect the misfeed and produce a feed stop command signal. The clutch/brake solenoid responsive to that signal takes from 20 milliseconds to 30 milliseconds to operate. Further, it takes from 100 milliseconds to 1 second for the clutch connected to the solenoid to disengage the flywheel from the crankshaft and for the brake connected to the crankshaft to bring the crankshaft to a stop. Using the minimums of the above times, if a misfeed is detected at the 3 o'clock downstroke reference point, it is not possible to stop the press operating at 120 strokes per minute before the moveable die reaches the 6 o'clock, bottom reference point which is necessary to prevent the production of a scrap workpiece and/or damage to the dies.